Typically, vehicles powered by internal combustion engines have an electric starter motor for starting the engine. The starting motor is electrically connected to a starting circuit which receives electrical energy from an electric storage battery. When an ignition keyswitch is operated, power from the battery is supplied to the starting motor to turn over the internal combustion engine. In common vehicle applications, devices such as engine control electronics, lighting systems, and vehicle accessories, which present an electrical load to the battery, or to alternator, when the vehicle engine is running.
Traditional batteries are often referred to as starting, lighting and ignition (SLI) batteries. In design and construction, these are multi-cell, lead-acid batteries, which are constructed from lead plates carrying active material and arranged into stacks. The stacks are inserted into partitioned cell compartments of a battery container, electrically connected, and flooded with dilute acid electrolyte.
Starting requires high power output for a short time period. SLI batteries of this construction are more than adequate for providing the relatively high power demand required for engine starting.
Maintaining electrical loads in the vehicle both during vehicle operation and during periods of non-operation requires a relatively lower power demand than starting. Therefore, SLI battery design is difficult as an SLI battery must be to optimized to perform, both short duration high-power output and long duration low-power output. An additional drawback of SLI batteries is relatively low specific energy (kilo-watt hours/grams (kWh/g)) as compared to other battery constructions owing to the weight of the lead plates and the liquid electrolyte.
More recently vehicle power systems have incorporated two batteries. A first battery in the system, a starting battery, is optimized for engine starting, that is, designed specifically for short duration, high-power output. A second battery in the system, a reserve battery, is optimized for operating and maintaining non-starting electrical loads. An advantage of such a system is that the starting battery may be made smaller and lighter yet capable of provide a high power output for a short period of time. In addition, the reserve battery may be made smaller and lighter, yet capable of satisfying the relatively low power requirements of the vehicle accessories. In combination, the two battery system can be designed to occupy less space and weigh less than a single traditional SLI battery.
Dual battery systems require control circuits to maintain the charge of both batteries in the system. Typically, the vehicle includes a regulation device which regulates the output of the alternator in response to the charging needs of the SLI battery and the vehicle electrical loads. In a dual battery system, each battery type delivers power and accepts charge at a different rate. For example, the starting battery delivers power at a very high rate and likewise accepts charge at a high rate. In contrast, the reserve battery delivers power at a lower rate and accepts charge at a lower rate. Each battery will typically exhibit a different state-of-charge, and hence require different charge maintenance. Additional advantages may also be attained by selectively coupling or decoupling the batteries during non-operational, starting and operational periods of the vehicle. However, careful management is required so as not to damage either the vehicle electrical system or the dual battery system.
Therefore, a dual-battery system for vehicle starting and operation that provides the advantages of reduced size and weight and includes power and charge management is needed.
Dougherty et al., U.S. Pat. No. 5,162,164, discloses a dual battery system in which two batteries are contained in a single casing. Thus, it should be understood that the identification of two batteries means, without limitation, containment in either separate casings or in one casing